Because of known receiver sensitivity and frequency selectivity advantages, there is a desire to employ coherent detection in optical receivers. Much greater sensitivity and frequency selectivity can be achieved by coherent detection than by the direct detection methods used in the prior art. Some coherent detection schemes have been proposed in the prior art.
In operating prior art coherent optical receivers, it has been difficult to tune a local oscillator laser to oscillate at a frequency that closely tracks whatever optical carrier frequency is received from a transmission medium. Since the development of a tunable distributed Bragg reflection laser, it is possible to tune a local oscillator laser through a wide frequency range and to track each of the channels of a large multi-channel received signal.
For some systems applications, it is desirable to employ several different optical carriers multiplexed together on a common optical media. Receivers in such systems need flexibility in their local oscillator frequency so that they can receive any one of the several different optical channels. In order to select those different optical channels, a coherent receiver must be arranged with a local oscillator laser that can be selectively tuned to any one optical frequency from a set of local oscillator frequencies, associated with the set of received optical carrier frequencies. Heretofore there has been no workable arrangement for generating a selectable set of local oscillator optical frequencies.
In addition to the problem of providing the selectable set of local oscillator frequencies, there is a further problem of compensating the laser bias current that is applied to the local oscillator for any drift which occurs in its operating characteristic as a result of aging, temperature change, or other changes of ambient conditions.